The classical models of estrogen receptor (ER) pharmacology held that they were two classes of ligands, agonists, which activate the receptor upon binding and antagonists which competitively block agonist access to the receptor. However, the observation that compounds, like tamoxifen and raloxifene, which were originally classified as antiestrogens are in fact Selective Estrogen Receptor Modulators (SERMs) manifesting their agonist or antagonist activities in a cell selective manner, suggests that ER-pharmacology is more complex. One approach we have taken to define the molecular basis of this selectivity has been to use phage display technology to develop a series of high affinity peptide probes whose ability to interact with ER is influenced by the nature of the bound ligand. Using these probes we demonstrated that estrogen altered the structure of ER facilitating the presentation of potential protein-protein interaction sites on the surface of the receptor. Surprisingly, we found that introduction of high affinity blocking peptides, derived from the sequences of the ER-interacting phage, had different affects on estradiol action in different target cells. We infer from these data that the mechanism by which estrogen displays its biological activity is not the same in all target cells. When these studies were extended to the study of SERMs, we observed that some surfaces on ER were presented in a compound specific manner. For instance, we identified sites on ER which were exposed in the presence of tamoxifen, but not in the presence of other structurally related compounds, like GW5638. These later sites on ER, are physiologically relevant as specific peptides which block these sites efficiently inhibit tamoxifen, but not estradiol, agonist activity in target cells. Cumulatively, these data indicate that ER-ligands manifest their tissue selective activities by facilitating the interaction of ER with different proteins or processes within the cell. Consequently, the overall objectives of the proposed project are; (1) Identification of peptide probes which distinguish between different ER-ligand complexes. (2) Pharmacological definition of the sites within ER which are presented upon binding different ligands and determination of their biological relevance. (3) Identification of co-activators which are utilized in a cell specific manner to facilitate estrogen responsiveness. (4) Validation of specific peptide-ER binding pockets implicated by our studies as targets for new drug discovery. We believe, that in addition to providing a molecular understanding of ER- pharmacology, the insights gained from these studies will be applicable to the progesterone and androgen receptors, where tissue selective agonists and antagonists would have clinical utility.